Cyclonic separation apparatus

ABSTRACT

The invention provides cyclonic separation apparatus containing a cyclone body having at least one fluid inlet and a fluid outlet, the fluid outlet being concentric with the longitudinal axis of the cyclone body. The cyclonic separation apparatus also contains a vortex finder projecting from an end surface of the cyclone body into the interior of the cyclonic separator, and a centerbody located partially within the vortex finder. The centerbody projects beyond the distal edge of the vortex finder so that the distance between the end surface of the cyclone body and the further end of the centerbody is at least twice the smallest diameter of the vortex finder, and the cross-sectional area of the centerbody is circular at any point along its length.

FIELD OF THE INVENTION

The invention relates to cyclonic separation apparatus, particularly butnot exclusively to cyclonic separation apparatus for use in a vacuumcleaner.

BACKGROUND OF THE INVENTION

Cyclonic separation apparatus consists generally of a frusto-conicalcyclone body having a tangential inlet at its larger, usually upper, endand a cone opening at its smaller, usually lower, end. A fluid carryingparticles entrained within it enters via the tangential inlet andfollows a helical path around the cyclone body. The particles areseparated out from the fluid during this motion and are carried ordropped through the cone opening into a collector from which they can bedisposed of as appropriate. The cleaned fluid, usually air, travelstowards the central axis of the cyclone body to form a vortex and exitsthe cyclonic separator via a vortex finder which is positioned at thelarger (upper) end of the cyclone body and is aligned with the centralaxis thereof.

The vortex finder usually takes the form of a simple tube extendingdownwardly into the cyclone body so that the vortex of exiting fluid isreliably directed out of the cyclone. However, the vortex finder has anumber of inherent disadvantages. One of these disadvantages is the factthat there is a significant pressure drop within the vortex finder dueto the high angular velocity of the exiting fluid. In an attempt toovercome this problem, centerbodies have been introduced into knownvortex finders in combination with tangential offtakes in order tostraighten the flow passing through and out of the cyclone. Someattempts have been made to reduce the swirl of the flow using fixedvanes. A variety of these attempts are illustrated in the paper entitled“The use of tangential offtakes for energy savings in processindustries” (T O'Doherty, M Biffin, N Syred: Journal of ProcessMechanical Engineering 1992, Vol 206). Other arrangements incorporatingcenterbodies or vanes are illustrated in WO 97/46323, WO 91/06750 andU.S. Pat. No. 5,444,982. In all of these pieces of prior art, thecenterbody is wholly contained within the vortex finder or, if it isnot, it projects only to a very minor extent into the cyclone body. Thisis because the single aim of the centerbody or vane is to remove theswirl from the flow within the vortex finder, rather than to stabilizeit.

Centerbodies have also been introduced to cyclonic separators for otherreasons. One such reason, illustrated in U.S. Pat. No. 4,278,452, is toexpand the outgoing fluid so that an outermost annulus of fluidcontaining any particles remaining entrained is recirculated through theseparator. However, by necessity, the major part of the centerbody mustremain outside the vortex finder and therefore is incapable ofstabilizing the fluid flow inside the vortex finder. Another use of acenterbody is to support an electrode by means of which a Coronadischarge is produced within the separation zone of the separator. Thisenhances the separation efficiency within the separation zone but,because the electrode must incorporate angular or pointed areas fromwhich the Corona will discharge, there can be no stabilization of theexiting fluid.

In CH 388267, use is made of a centerbody projecting out of a vortexfinder to prevent bubbles of gas escaping from the main outlet ofapparatus for separating solid particles and gas bubbles from a liquidsuspension. The centerbody has an essentially flat end. The gas bubbles,which migrate to the vortex core during operation, are caused to exitthe apparatus via the cone opening, which forms an outlet for thecyclone.

Another problem associated with vortex finders is the fact that, duringoperation of the cyclonic separation apparatus, the vortex coreprecesses around the interior of the vortex finder causing a significantamount of noise. The provision of a centerbody wholly within the vortexfinder has been recognized as contributing to the reduction of the noiseassociated with the exiting fluid to a certain extent but no attempt hasbeen made to make use of a centerbody to reduce the noise still further.

SUMMARY OF THE INVENTION

In domestic appliances such as vacuum cleaners, noise is alwaysundesirable and there is an ongoing desire to reduce the noiseassociated with the appliance as far as possible. It is therefore anobject of the present invention to provide cyclonic separationapparatus, suitable for incorporation into a domestic appliance, inwhich the noise level is improved. It is a further object of theinvention to provide cyclonic separation apparatus in which the pressuredrop appearing across the vortex finder is as small as possible. It is astill further object of the invention to provide cyclonic separationapparatus suitable for use in a domestic vacuum cleaner.

The invention provides cyclonic separation apparatus containing acyclone body having at least one fluid inlet and a fluid outlet having avortex finder. The invention also provides a vacuum cleanerincorporating such cyclonic separation apparatus. Further and preferredfeatures of the cyclonic separation apparatus include a centerbodyhaving a circular cross-section and a hemispherical, conical orfrusto-conical end which protrudes beyond the lowermost end of thevortex finder to a distance at which the furthermost end of thecenterbody is at least twice the smallest diameter of the vortex finderfrom the end surface of the cyclone body reduces the noise associatedwith the exiting vortex to an appreciable degree. The reduction has beenfound to be significantly better than in the case when the vortex finderdoes not protrude out of the vortex finder to any significant extent. Itis believed that precession of the vortex core when bounded by the wallsof the vortex finder causes pressure perturbations within the airflowwhich are manifested as noise. Hence it is desirable to stabilize thisrotation completely before the exiting air enters the vortex finder. Theextension of the centerbody into the core's low pressure area before itreaches the vortex finder causes the core to stabilize before it reachesthe vortex finder. The noise level is thereby reduced. Experimentationwith specific apparatus has shown that, for specific dimensions ofcyclone, vortex finder and centerbody, there are optimum distances fromthe upper surface of the cyclone to which the centerbody must extend. Itwill be clear from the description and examples which follow that it isnot necessary for the centerbody to extend all the way up the vortexfinder to the upper surface of the cyclone.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, wherein:

FIG. 1 shows, in cross section, cyclonic separation apparatus accordingto the present invention and suitable for use in a vacuum cleaner;

FIG. 2a shows, to a larger scale, the centerbody forming part of theapparatus shown in FIG. 1;

FIG. 2b shows a first alternative configuration of the centerbody ofFIG. 2a;

FIG. 2c shows a second alternative configuration of the centerbody ofFIG. 2a;

FIG. 2d shows a second alternative configuration of the centerbody ofFIG. 2a.

FIG. 3 is a cross-section through part of alternative cyclonicseparation apparatus according to the present invention;

FIG. 4 is a schematic drawing of the test apparatus used to determinethe results of the experiments described below; and

FIG. 5 is a graph showing a comparison in cyclone noise with and withoutan optimised vortex finder centerbody in place.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows cyclonic separation apparatus 10 suitable for use in acyclonic vacuum cleaner. In fact, in this example, the cyclonicseparation apparatus consists of two concentric cyclones 12,14 forsequential cleaning of an airflow. The remaining features of the vacuumcleaner (such as the cleaner head or hose, the motor, motor filters,handle, supporting wheels, etc.) are not shown in the drawing becausethey do not form part of the present invention and will not be describedany further here. Indeed, it is only the innermost, high efficiencycyclone 14 which incorporates a vortex finder in this embodiment andtherefore it is only the innermost cyclone 14 which is of interest inthe context of this invention. It will, however, be understood that theinvention is applicable to cyclonic separation apparatus other than thatwhich is suitable for use in vacuum cleaners and also to cyclonicseparation apparatus incorporating only a single cyclone.

The innermost cyclone 14 comprise a cyclone body 16 which is generallyfrusto-conical in shape and has a fluid inlet 18 at its upper end and acone opening 20 at its lower end. The cone opening 20 is surrounded by aclosed collection chamber 22 in which particles entering the cyclone 14via the fluid inlet 18 and separated from the airflow within the cyclonebody 16 are collected. The cyclone body 16 has an upper surface 24 inthe centre of which is located a vortex finder 26. The vortex finder isgenerally tubular in shape and has a lower cylindrical portion 26 awhich merges into an upper frusto-conical portion 26 b which leads outof the cyclone body 16 to an exit conduit. The operation of cyclonicseparation apparatus of the type described thus far is well known anddocumented elsewhere and will not be described in any further detailhere.

The invention takes the form of a vortex finder centerbody 30 which islocated inside the vortex finder 26 and is shown in position in FIG. 1.The centerbody 30 is also shown on an enlarged scale in FIG. 2a. Thecenterbody 30 comprises a central elongate member 32 which iscylindrical along the majority of its length and has hemispherical ends32 a, 32 b. The hemispherical shaping of the ends 32 a, 32 b reduces therisk of turbulence being introduced to the airflow as a result of thepresence of the centerbody 30. The elongate member 32 carries twodiametrically opposed tabs 34 which are generally rectangular in shapeand extend radially outwardly from the elongate member 32 sufficientlyfar to abut against the interior walls of the vortex finder 26 withinthe cylindrical portion 26 a. The downstream edges of the tabs 34 haveradiussed outer corners to reduce the risk of turbulence beingintroduced. Also, notches or grooves 36 a are formed in the outer edgesof the tabs 34 whilst corresponding tongues or projections 36 b areformed in the interior walls of the cylindrical portion 26 a of thevortex finder 26. The tongues or projections 36b are also diametricallyopposed and are designed and positioned to cooperate with the notches orgrooves 36 a in the tabs 34 and so hold the centerbody 30 in position inthe vortex finder 26. It will be understood that the exact method ofholding the centerbody in position is immaterial to the invention andthe notches/grooves 36 a and tongues/projections 36 b can be replaced byany alternative suitable means for reliably holding the centerbody 30within the vortex finder 26 so that the centerbody 30 will not bedislodged by the likely rate of flow of fluid through the cyclonicseparation apparatus, nor subjected to unacceptable vibrations. A snapfitting method is regarded as particularly desirable because of its easeof manufacture and ease of use.

The length of the centerbody 30 and its positioning are sufficient toensure that the end 32 a of the centerbody 30 furthest from the uppersurface 24 lies at a point whose distance below the upper surface 24 isequal to at least twice the smallest diameter of the vortex finder 26.Thus the length of the protrusion of the centerbody 30 beyond the lowerend of the vortex finder 26 added to the total length of the vortexfinder 26 (below the upper surface 24) must be at least twice thediameter of the vortex finder 26. If this criterion is satisfied, thenoise reduction achievable is improved. In the embodiment shown in FIG.1, the lowermost point of the centerbody 30 lies below the upper surface24 at a distance which is equal to approximately 2.58 times the smallestdiameter of the vortex finder 26. Specifically, the lowermost point ofthe centerbody 30 lies 82.5 mm below the upper surface 24 and thesmallest diameter of the vortex finder 26 is 32 mm. Furthermore, thelength of the centerbody 30 is 60 mm and its diameter is 6 mm. Thecenterbody 30 projects below the lowermost edge of the vortex finder 26to a distance of 16.5 mm. This arrangement succeeds in achieving areduction in overall sound pressure level (noise) emitted from the wholevacuum cleaner product of 1.5 dBA.

In order for the centerbody 30 to function well, the cross-section ofthe centerbody 30 is made circular at any point along its length. Themain body of the centerbody 30 is cylindrical, as mentioned above, butthe upstream and downstream ends 32 a, 32 b can take various shapes. Inthe embodiment shown in FIG. 2a, both of the ends 32 a, 32 b arehemispherical. However, one or other of the ends could be, for example,conical or frusto-conical, although a conical end will be preferablebecause this will reduce pressure drop and/or energy losses within theapparatus. An alternative centerbody 50 is shown in FIG. 2b in which thecentral portion of the elongate body 52 of the centerbody 50 is againcylindrical and the downstream end 52 b is hemispherical, but theupstream end 52 a is conical in shape. A further difference between thecenterbody 50 shown in FIG. 2a and the alternative centerbody shown inFIG. 2b is the number of tabs 54 provided on the elongate body 52 forsupport purposes. In the embodiment shown in FIG. 2b, four equiangularlyspaced tabs 54 are provided. Corresonding tongues are then provided onthe wall of the vortex finder 26 in order to support the centerbody 50therein.

A further alternative embodiment is shown from two different angles inFIG. 2c. In the Figure, the centerbody 70 is shown from two differentperspective views so that the helical shape of the tabs 74 can clearlybe seen. The helical shape is present so that the tabs 74 do notinterfere with the rotational motion of the air exiting via the vortexfinder. As in the embodiment shown in FIG. 2a, the elongate body 72 isgenerally cylindrical in shape and the upstream end 72 a ishemispherical. The downstream end 72 b is planar. Each tab 74 is shapedat its distal end so as to include grooves 74 a which cooperate withprojections moulded into the vortex finder so that the centerbody 70 isheld firmly in the correct position in the vortex finder.

An alternative configuration of separation apparatus is shown in part inFIG. 3. The figure shows only the upper portion of the separationapparatus 80 which, as before, comprises an upstream, low-efficiencycyclone 82 and a downstream, high-efficiency cyclone 84. Thelow-efficiency cyclone 84 has a cyclone body 86 which has an inlet 88communicating with the upper end of the cyclone 84 and a cone opening(not shown) at the opposite end thereof surrounded by a collector (alsonot shown) in the same manner as shown in FIG. 1. The cyclone 84 isclosed at its upper end by an upper surface 90 from which depends avortex finder 92 which extends into the interior of the cyclone 84 alonga central axis thereof. The vortex finder 92 is cylindrical in shape forthe majority of its length but flares outwardly at its upper end so asto merge smoothly with the upper surface 90.

A centerbody 94 is immovably mounted within the vortex finder 92 andextends from a point above the level of the upper surface 90 rightthrough the vortex finder 92 so that the centerbody 94 projects beyondthe lower edge of the vortex finder 92. The body of the centerbody 94 isgenerally cylindrical with a slight taper towards the upstream end 94 b.The upstream end 94 a is hemispherical in shape but its downstream end94 b is merely planar. The centerbody 94 has three equiangularly spacedtabs or flanges 96 which extend outwardly from the upper end of thecenterbody 94 to the inner wall of the vortex finder 92. The outermostedges of the tabs or flanges 96 are shaped so as to follow the shape ofthe inner wall of the vortex finder 92 to assist with correctpositioning of the centerbody 94.

In this embodiment, the diameter of the centerbody 94 is 10 mm and thediameter D1 of the vortex finder 92 is 30.3 mm. The length L1 of thevortex finder is 50 mm and the distance L2 between the lower end 94 a ofthe centerbody 94 and the upper surface 90 is 64.4 mm. Hence thelowermost point of the centerbody 94 lies below the upper surface 90 ata distance of 2.13 times the (smallest) diameter of the vortex finder92. The centerbody 94 projects below the vortex finder 92 to a distanceof 14.4 mm.

This invention will be better understood with reference to the followingexamples which are intended to illustrate specific embodiments withinthe overall scope of the invention as claimed.

Tests to determine the optimum position of the lowermost end of thecenterbody in the apparatus shown in FIG. 1 have been carried out. Thetest method and apparatus will now be described with reference to FIG. 4of the accompanying drawings.

A clear cyclone 100 with a variable-length vortex finder 120 and avariable-length centerbody 140 was mounted in an upright position usingappropriate clamps and mounting devices (not shown). The cyclone 100 hada maximum diameter of 140 mm and a height of 360 mm. Suction wasprovided to the cyclone 100 by a quiet source connected via a firstflexible hose 102 to ensure the minimum of interference from motornoise. A second flexible hose 104 connected to the cyclone inlet 106took incoming air from a remote chamber (not shown) to avoidinterference from the noise associated with air entering the hoseopening. At the inlet 106 to the cyclone 100 a flow rate meter 108 wasattached to allow the incoming flow rate to be measured accurately.

The variable-length vortex finder 120 consisted of a tube 122 of fixedlength and fixed. diameter connected to the first flexible hose 102 andslidably mounted in the upper plate 110 of the cyclone 100 by means of asealing and clamping ring 124. In this case, the diameter of the tubewas 32 mm. By clamping the tube 122 at different positions so that itprojected into the cyclone 100 by different amounts, the length S of thevortex finder 120 could be varied. The variable-length centerbody 140consisted of an elongate member 142 mounted in a knee 126 in the upperend of the vortex finder 120. The elongate member 142 was slidablymounted in the knee 126 by means of a sealing and clamping block 144.Further support was provided to the elongate member 142 by way of twotabs 146 extending from the elongate member 142 to the interior wall ofthe vortex finder 122. The tabs 146 prevented the elongate member 142from oscillating during the test procedure. By clamping the elongatemember 142 so that it projected beyond the lower end 128 of the tube 122by different amounts, the length L of the centerbody 140 could bevaried.

In order to perform the experiment, the vortex finder length S was setto the required value and the end of the elongate member 142 was setflush with the lower end 128 of the tube 122 (ie, L=0). The suctionsource was activated and the flow rate measured and set to the requiredlevel by appropriate adjustment The centerbody 140 was then moved downin 5 mm stages and sound measurements taken at each stage. The optimumlength of the centerbody being sought was the length at which the noiselevel was reduced to a minimum. When an approximate location of theoptimum length of the centerbody 140 had been located 2 mm increments incenterbody length L were then used to pinpoint more accurately theoptimum length.

Having determined the optimum length of the centerbody 140 for a givenflowrate and a given vortex finder length S, the flowrate was thenvaried by adjusting the suction source and the incremental variation ofthe centerbody length L was repeated to determine the optimum centerbodylength for that flowrate. Having determined the optimum centerbodylength for each required flowrate and a given vortex finder length, thevortex finder length was then adjusted and a second series ofexperiments were carried out using the same set of flowrates to producecomparable results. The results obtained are set out below.

Flow Rate Vortex Finder Length Optimum Centerbody Length L(liters/second) S (mm) (mm) 20 66 20 22.5 66 22 25 66 23 20 40 45 22.540 55 25 40 49 20 80 10 22.5 80 6 25 80 25

The optimum length was further defined as being the length of thecenterbody at which noise reduction reversed to a slight gain in noiselevel. The optimum length was therefore seen as a minimum overall soundpressure level, a point where no significant reduction is gained bycontinuing to extend the centerbody or a point where the tonal qualitystarts to deteriorate. In particular the fundamental frequency,identified using narrow band analysis, of the vortex precession wasconsidered as being at its minimum at the optimum length.

Further tests revealed that, in a cyclone body having diameter of 140mm, a height of 300 mm, a vortex finder diameter of 32 mm and a vortexfinder length of 66 mm, the optimum protrusion of the centerbody 30beyond the lowermost end of the vortex finder is 16.5 mm. This gives adistance between the lowermost end of the centerbody 30 and the uppersurface 24 of 82.5 mm, which is 2.58 times the diameter of the vortexfinder 26.

Further tests were carried out using apparatus similar to that describedabove but with replaceable vortex finders having different diameters. Ineach case, the vortex finder length was 46 mm and a fixed flow rate of27 litres/second was used. The centerbody used was similar to thatdescribed above but had a diameter of 10 mm. A method similar to thatdescribed above was used to find the optimum centerbody length for eachvortex finder diameter. The results obtained are as follows:

Vortex Finder Diameter Optimum Centerbody Length D1 (mm) L1 (mm) 38 8534 88 30 76 28 64 26 61

This clearly shows that the optimum centerbody length for a given flowrate and a given centerbody diameter decreases generally with thediameter of the vortex finder.

The centerbody 30 is preferably made from a plastics material and mustbe sufficiently rigid not to bend or oscillate when exposed to theflowrates likely to be passed through the separation apparatus. For acenterbody suitable for use in a vacuum cleaner, a suitable material ispolypropylene and this allows the centerbody to be moulded simply andeconomically using any one of a variety of common techniques, forexample, injection moulding.

Testing and research have shown that, depending upon the specificconfiguration of the cyclone, optimising the centerbody length canresult in a reduction of between 2 and 6 dB of the overall soundpressure level of a cyclone. This is sufficient to achieve an audibledifference in the overall noise levels of a domestic vacuum cleaner.FIG. 5 illustrates the difference in noise (sound pressure level)produced by the cyclone of a specific vacuum cleaner with and without anoptimised centerbody in place. As can clearly be seen, the presence ofthe centerbody (noise level shown in bold lines) removes a significanttone which is present when the centerbody is absent (noise level shownin dotted lines). The advantages of reducing the noise level of adomestic vacuum cleaner are to improve consumer satisfaction and allow auser to hear other sounds and noises within the environment in which thecleaner is being used. This can improve the safety of the user whenusing the cleaner.

What is claimed is:
 1. A cyclonic separation apparatus comprising a cyclone body having at least one fluid inlet and a fluid outlet, the fluid outlet being concentric with a longitudinal axis of the cyclone body and comprising a vortex finder projecting from an end surface of the cyclone body into the interior thereof, and a centerbody located partially within the vortex finder and projecting beyond the end thereof opposite the end surface so that the distance between the end surface of the cyclone body and the furthermost end of the centerbody is at least twice the smallest diameter of the vortex finder, the cross-sectional area of the centerbody being circular at any point along its length, wherein the centerbody tapers inwardly towards its furthermost end and is hemispherical, conical or frusto-conical in shape.
 2. A cyclonic separation apparatus as claimed in claim 1, wherein the distance between the end surface of the cyclone body and the furthermost end of the centerbody is at least 2.3 times the smallest diameter of the vortex finder.
 3. A cyclonic separation apparatus as claimed in claim 2, wherein the distance between the end surface of the cyclone body and the furthermost end of the centerbody is at least 2.5 times the smallest diameter of the vortex finder.
 4. A cyclonic separation apparatus as claimed in claim 1, wherein the centerbody is generally cylindrical with at least one hemispherical end.
 5. A cyclonic separation apparatus as claimed in claim 1; wherein the centerbody is generally cylindrical with at least one conical end.
 6. A cyclonic separation apparatus as claimed in claim 1, wherein the diameter of the centerbody is no more than one half of the smallest diameter of the vortex finder.
 7. A cyclonic separation apparatus as claimed in claim 6, wherein the diameter of the centerbody is no more than one third of the smallest diameter of the vortex finder.
 8. A cyclonic separation apparatus as claimed in claim 7, wherein the smallest diameter of the vortex finder is about 32 mm and the diameter of the centerbody is about 6 mm.
 9. A cyclonic separation apparatus as claimed in claim 8, wherein the distance of the furthermost end of the centerbody is between 80 mm and 110 mm from the end surface of the cyclone body.
 10. A cyclonic separation apparatus as claimed in claim 9, wherein the distance of the furthermost end of the centerbody is between 85 mm and 95 mm from the end surface of the cyclone body.
 11. A cyclonic separation apparatus as claimed in claim 7, wherein the smallest diameter of the vortex finder is about 30 mm and the diameter of the centerbody is about 10 mm.
 12. A cyclonic separation apparatus as claimed in claim 11, wherein the distance of the furthermost end of the centerbody is between 50 mm and 90 mm from the end surface of the cyclone body.
 13. A cyclonic separation apparatus as claimed in claim 12, wherein the distance of the furthermost end of the centerbody is between 60 mm and 70 mm from the end surface of the cyclone body.
 14. A cyclonic separation apparatus as claimed in claim 1, wherein the centerbody projects beyond a lower edge of the vortex finder to a distance of at least 10 mm.
 15. A cyclonic separation apparatus as claimed in claim 14, wherein the centerbody projects beyond the lower edge of the vortex finder to a distance of about 14.4 mm.
 16. A cyclonic separation apparatus as claimed in claim 14, wherein the centerbody projects beyond the lower edge of the vortex finder to a distance of about 16.5 mm.
 17. A cyclonic separation apparatus as claimed in claim 1, wherein the centerbody is supported in the vortex finder by supporting tabs extending as far as an interior wall of the vortex finder.
 18. A cyclonic separation apparatus as claimed in claim 17, wherein the tabs are diametrically opposed.
 19. A cyclonic separation apparatus as claimed in claim 17, wherein the tabs comprise helical vanes.
 20. A cyclonic separation apparatus as claimed in claim 17, wherein the tabs and the interior wall of the vortex finder incorporate retaining means for retaining the centerbody in position inside the vortex finder.
 21. A cyclonic separation apparatus as claimed in claim 20, wherein the retaining means comprises resilient tongues engageable with corresponding grooves.
 22. A vacuum cleaner incorporating a cyclonic separation apparatus comprising a cyclone body having at least one fluid inlet and a fluid outlet, the fluid outlet being concentric with a longitudinal axis of the cyclone body and comprising a vortex finder projecting from an end surface of the cyclone body into the interior thereof, and a centerbody located partially within the vortex finder and projecting beyond the end thereof opposite the end surface so that the distance between the end surface of the cyclone body and the furthermost end of the centerbody is at least twice the smallest diameter of the vortex finder, the cross-sectional area of the centerbody being circular at any point along its length, wherein the centerbody tapers inwardly towards its furthermost end and is hemispherical, conical or frusto-conical in shape. 